1. Field of the Invention
The present invention relates to beads or small balls of ceramic material for use inter alia in methods and devices for micro-crushing and for producing a dispersion in a moist medium.
2. Description of the Prior Art
In recent years there has been a spectacular development in devices and methods for micro-crushing and for producing a dispersion in a moist medium. The devices (bead-mills) and methods use dispersing or crushing agents which are spherical and have a small diameter (e.g. 0.4 to 4 mm) and are used, e.g., to disperse the constituents of paints, varnishes, inks or the like introduced in liquid or solid form, or to micronise a powder in a moist medium, the powder having been pre-crushed by conventional methods (e.g. in a ball-mill).
Briefly, dispersion or crushing agents must have the following properties:
Mechanical resistance to crushing and impacts; PA1 Resistance to abrasion; and PA1 A particle size suitable for the application.
In addition, a high density and the property of not coloring the processed products are important in certain applications, e.g., in the treatment of slips or very viscous mixtures in the first case and the treatment of white paints or powders in the second case.
The original dispersing agent used in these devices and methods was sand, e.g., Ottawa sand having round grains. If this natural product is used, however, the devices are not as efficient as they might be, since it is available only within a limited range of particle sizes (0.4-0.8mm) and its resistance to abrasion is relatively low, which limits its use to the treatment of relatively soft powder. Attempts have therefore been made to develop dispersing or crushing agents which are more suited to requirements than sand is. For example, glass beads and sintered ceramic beads are at present commercially available.
Glass beads are available within a wide range of diameters and give satisfactory results when the substances processed are not very abrasive. However, they have little resistance to abrasion and have relatively low density (approx. 2.6), and are therefore unsuitable for processing excessively viscous or abrasive products. They also have low resistance to impact and are progressively destroyed by flaking or even splitting. New beads have good resistance to crushing, but during use this property is rapidly lost as a result of microscopic fissures, scratches or surface splitting.
Beads of ceramics sintered at high temperature, more particularly titanium dioxide or alumina, have better resistance to abrasion than glass beads, but the resistance is still limited by the quality of sintering, i.e., the quality of the intercrystalline bonds. The method of manufacturing these sintered beads is still secret.
There is thus a need for beads having further improved properties.
U.S. Pat. No. 2,924,533 discloses spherical particles made of crystalline zirconium dioxide with or without crystalline mullite embedded in a vitreous silica-containing material containing, by weight, 22.5-75% zirconium dioxide, 22.5-55% silica and 0-22.5% alumina and having good mechanical strength. These particles are described as of use as blasting agents (sanding).
The invention relates to higher-quality ceramic beads for use inter alia as dispersion or crushing agents.